Conventionally, there has been known an ink-jet recording device which includes an ink-jet head having a plurality of nozzles for ejecting droplets of ink material.
Also, another ink-jet recording device for color printing has been used in which a plurality of ink-jet heads have ink materials of different colors, respectively. The ink-jet heads are supported on a carriage which moves transversely on a recording medium such as plain paper. With this color ink-jet recording device, the plurality of color ink materials are ejected from respective heads with the movement of the carriage, thereby forming a color image on the recording medium.
To form a high quality color image on the recording medium using such an ink-jet color recording device, each ink droplet ejected from the nozzles must be deposited in positions on the recording medium for reproducing the original colors thereon. For this purpose, the nozzles should be formed in respective positions in each head. Further, the heads should be mounted in positions with great precision on the carriage while maintaining a precise positional relationship of the plurality of heads. Furthermore, when the plurality of heads are mounted on the carriage through a common plate, the common plate should be secured in position on the carriage by the use of a suitable tool. As a result, the assembling of the ink-jet recording head presents significant difficulties.
In addition, the carriage is typically designed to travel along a guide rod extending in the transverse direction of the recording medium. With this arrangement, the carriage may slightly incline with respect to the guide rod due to a variation of the frictional force to be possibly generated between the guide rod and the carriage, which may result in an unwanted positional displacement of the deposited ink droplets on the recording medium. This may also happen due to a wearing of the contact portions of the carriage and the guide rod.
Another ink-jet recording head that is known in the art has a plurality of nozzles for ejecting ink droplets of different sizes to form respective ink dots having different diameters on the recording medium, thereby reproducing a halftone image by the combination of large and small ink dots.
With this ink-jet head, by using three or four ink materials having different colors, respectively, a variety of colors can be reproduced on the recording medium by depositing two ink dots having different sizes and colors on the same place or by depositing the ink dots on different places in a small area and thereby visually mixing different colors. It is to be noted that the former color mixing method is more advantageous than the latter one for reproducing halftone colors.
One method for ejecting different sizes of ink droplets is accomplished by providing the ink-jet recording head with nozzles of different sizes. For example, U.S. Pat. No. 5,208,605 discloses an ink-jet recording head which includes a plurality of small nozzles arranged on a first line and a plurality of large nozzles arranged on a second line parallel to the first one. Also, U.S. Pat. No. 5,412,410 discloses another ink-jet recording head in which the large and small nozzles are arranged alternately on a line extending in a direction along which the recording medium is forwarded.
However, each of the ink-jet recording heads disclosed has drawbacks. For example, when superimposing the small ink dot on the large ink dot, the second ink dot can be deposited on the first ink dot which has not been dried yet, which eventually results in an unwanted color mixing and therefore it is impossible to reproduce an intended color. In particular, in color reproduction, the mixing of color is a critical problem which will significantly degrade the quality of the reproduced image.